Process for improving elevated temperature strength and ductility of nickel-base alloys



United States Patent 3,512,963 PROCESS FOR IMPROVING ELEVATED TEMPERA-TURE STRENGTH AND DUCTILITY OF NICKEL- BASE ALLOYS Jacob Schramm,Ringwood, and John H. Olson, Franklin Lakes, N.J., and Clarence G.Bieber, Sulfern, N.Y., assignors to The International Nickel Company,Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed July25, 1966, Ser. No. 567,400 Int. Cl. C22c 19/00, 1/02 US. Cl. 75-171ABSTRACT OF THE DISCLOSURE The elevated temperature properties and hotworkability of nickel-base alloys such as nickel-chromium alloys ismarkedly improved by a melting practice involving vacuum treatmentfollowed by an addition of magnesium under a non-reactive gasatmosphere.

The present invention is directed to a method for inlproving theproperties of nickel-base alloys and, more particularly, to a method forimproving the elevated temperature strength and ductility thereof.

The beneficial effects which flow from vacuum melting and vacumtreatment of molten nickel-base alloys from the viewpoint of improvingthe properties thereof are now well known. In fact, for the melting ofmany alloys which are required to have high strength and reproducibleproperties at elevated temperatures this is a standard practice. The useof high purity charges and improved vacuum melting techniques hasresulted in very low impurity 6 Claims ice reducing the rate ofmagnesium volatilization, providing practical control of final magnesiumcontent, etc., a nonreactive gas such as argon, helium or nitrogenhaving a partial pressure in the amount of at least about one-thirdatmosphere up to about one atmosphere is placed over the melt uponcompletion of the vacuum melting cycle,

magnesium is then introduced into the melt, and the magnesium-containingmelt is thereafter cast. Magnesium may conveniently be introduced intothe melt in the form of a master alloy containing about 4% to aboutmagnesium, e.g., about 12% to 14% magnesium, and the balance essentiallynickel. The nonreactive gas can be led directly into the vacuum meltingchamber or vacuum melted metal can be carried in a ladle from the vacuummelting chamber through a vacuum lock into an inert-gas chamber for themagnesium addition.

Alloys susceptible to treatment in accordance with the invention maycontain, by weight, up to chromium, e.g., about 10% to about 22%chromium, up to cobalt, e.g., about 8% to about 22% cobalt, up to about15% molybdenum, e.g., about 2% to about 12% molybdenum, up to 5%titanium, e.g., about 0.25% to about 5% titanium, up to 7% aluminum,e.g., about 0.25% to about 7% aluminum, up to about 15% tungsten, up to0.2% carbon, e.g., about 0.02% to about 0.2% carbon, up to about 10%columbium, e.g., about 0.5% to about 8% columbium, up to about iron, upto about 0.1% boron, up to about 0.25 zirconium, up to about 2%manganese, up to about 2% silicon, and the balance essentially nickel.The sulfur content of the alloys should be as low as possible.Compositions of various alloys susceptible to treatment in accordancewith the invention are set forth in the following Table I.

Bal. ess.- Bala11 ce essentially, including minor amounts of manganese,silicon, borozirconium, impurities, etc.

levels in vacuum melted products. Thus, sulfur levels not exceedingabout 0.005% in vacuum melted alloys is common.

We have now discovered a method wherein vacuum melting is employed andwherein the properties of nickelbase alloys treated in accordance withthe method are improved as compared to alloys prepared by vacuum meltingand/or vacuum treating alone.

It is an object of the present invention to provide a process forimproving the elevated temperature strength and ductility of nickel-basealloys.

It is a further object of the invention to provide a process forimproving the hot workability of nickel-base alloys.

Other objects and advantages of the invention will be- 7 It is foundthat processing in accordance with the invention to provide a controlledretained magnesium content in vacuum melted alloys even though contentsof sulfur and other impurities are very low provides markedly improvedrupture life and rupture ductility in the treated alloys, e.g.,nickel-chromium alloys. Furthermore, hot workability of the treatedalloys is greatly improved.

In order to give those skilled in the art a better understanding andappreciation of the advantages of the invention, the followingillustrative examples are given.

EXAMPLE I A series of age-hardenable alloys containing nominally about21.5% chromium, about 13.5% cobalt, about 4% molybdenum, about 2%columbium, about 2.5% titanium, about 0.20% aluminum, about 0.05%carbon, not more than 0.005% sulfur, and the balance essentially nickelwere prepared by vacuum melting under a pressure not exceeding 10microns of mercury. In each instance, the molten metal was treated withabout 0.05% carbon, and held in vacuum about 30 minutes at 2900" F. toremove oxygen and other gasses prior to magnesium treat ment. The alloyswere worked from the ingot stage to %-ll1Ch diameter hot rolled rodusing the same hot working procedure in each instance. The hot rolledrod material was annealed for one hour at 1800 F. followed by aircooling and a 24 hour aging at 1400 F. It was found that the alloys towhich no magnesium had been added had rupture lives when tested at 1200F. under a stress of 100,000 pounds per square inch (p.s.i.) in therange from about 122 hours to about 188 hours with elongations in therange of 9.5% to 17% and reductions in area in the range of about 11% to30%. Other vacuum melted alloys in the series to which magnesium wasadded under a partial pressure of about one-half atmosphere of argon inaccordance with the invention had magnesium contents, and when testedunder the same stress rupture conditions, had rupture lives andductility values as set forth in the following Table II.

TABLE II Magnesium Rupture Reduction in content, life, Elongation, area,percent hours percent percent EXAMPLE II A number of vacuum meltedingots were prepared from an alloy containing nominally about 0.05%carbon, about 15% chromium, about 3% molybdenum, about 3% tungsten,about 3% columbium, about 7% iron, about 0.4% aluminum, about 0.5%titanium, not more than about 0.004% sulfur, and the balance essentiallynickel. A vacuum not exceeding about 10 microms of mercury was employedin each instance for melting. In each instance, the molten metal wastreated with about 0.05% carbon and held in vacuum for about 30 minutesat 2900 F. to remove oxygen and other gases prior to magnesiumtreatment. One of the ingots was cast without a magnesium addition.Another melt was treated with magnesium under vacuum and contained only0.005% magnesium. Magnesium additions were made to the remainder of thevacuum melts under argon as in Example I and magnesium was recoveredtherein in amounts up to 0.035%. The alloys contained about 0.005% boronand about 0.02% zirconium. The resulting ingots were converted to hotrolled square bar 0.64 inch square using the same procedure in eachinstance. Material from each of the bars was annealed at 1800 F. for onehour, air cooled and subjected to stress rupture at 1200 F. and 70,000psi. with the results set forth in the following Table III.

TABLE III Magnesium con- Rupture Elonga- Reduction Alloy tent, life,tion, in area, Hot No. percent hours percent percent workability 6 18530 32 Very poor.

I 0.005 277 26 24 Good.

O. 017 660 58 65 D0. 0.022 299 60 69 Do. 0. 026 530 45 69 D0. 0.035 1,015 33 70 Do.

It is to be noted that recovery of magnesium in the material produced asubstantial increase of rupture life together with marked improvement inductility values. It is further to be noted that even treated withmagnesium under vacuum wherein only 0.005 magnesium Was recovered in thealloy improved rupture life and hot workability. Alloy No. 6 to which onmagnesium was added contained 1 part per million (p.p.m.) hydrogen, 6p.p.m. oxygen, and less than 5 p.p.m. nitrogen. The gas contents ofAlloy No. 11, which contained 0.035% magnesium, were 1.1 p.p.m.hydrogen, 6 p.p.m. oxygen, and less than 5 p.p.m. nitrogen, i.e., werealmost axactly the same as for Alloy No 6. Microexamination of thestress-rupture specimens of Alloys Nos. 6 and 7 showed that grain 4boundary microcracking occurred therein with little grain deformationwhereas in Alloys Nos. 8 to 11 considerable grain deformation occurredbefore grain boundary cracking took place.

The invention is applicable to the production of castings as well as tothe production of wrought products.

It is to be understood that by the term vacuum melting as employedherein, melting under a vacuum not exceeding about 10 microns of mercurypressure is intended. Satisfactory results are also achieved inaccordance with the invention when air melted metal is subjected to avacuum treatment for at least about 5 minutes at a pressure notexceeding 10 microns of mercury pressure followed by magnesium treatmentunder a nonreactive gas at a pressure of about one-third to about oneatmosphere.

It is further to be understood that the processing according to theinvention is particularly applicable to the processing of alloyscontaining about 14% to 16% chromium, about 2.75 to 3.25 columbium (plustantalum), about 2.75% to 3.25 tungsten, about 2.75 to 3.25% molybdenum,about 5% to about 9% iron, up to about 0.08% carbon, about 0.3% to about0.6% aluminum, about 0.4% to about 0.7% titanium, about 0.003% to about0.008% boron, about 0.01% to about 0.05% zirconium, up to about 0.05magnesium, up to about 0.75% manganese, up to about 0.4% silicon, withthe balance nickel, and of age-hardenable alloys containing about 20% toabout 22% chromium, about 1.5% to 2.5% columbium (plus tantalum), about3.5% to 4.5% molybdenum, up to about 2% iron, up to about 0.07% carbon,about 0.2% to about 0.3% aluminum, about 2.3% to 3% titanium, about0.002% to 0.008% boron, about 0.02% to about 0.08% zirconium, up toabout 0.05 magnesium, up to about 0.25% manganese, up to about 0.15%silicon, with the balance essentially nickel.

We claim:

1. The process for improving the rupture life and ductility ofnickel-chromium alloys which comprises subjecting said alloy in themolten condition to a vacuum treatment to reduce the impurity contentsthereof to low levels and thereafter introducing magnesium into thevacuum treated melt such that the magnesium introduction step is carriedout under a nonreactive gas atmosphere having a partial pressure ofabout one-third to about one atmosphere and casting the thus-treatedalloy containing at least about 0.005 and up to about 0.1% magnesium.

2. The process according to claim 1 wherein the nonreactive gas isselected from the group consisting of argon, helium and nitrogen.

3. The process according to claim 1 wherein at least about 0.015%magnesium is retained in the treated alloy.

4. The process according to claim 1 wherein the alloy contains about 10%to about 25% chromium, up to about 15% molybdenum, up to about 15%tungsten, up to about 8% columbium, up to about 7% aluminum, up to about7% titanium, up to about cobalt, up to about 0.1% boron, up to about0.25 zirconium, up to about iron, not more than 0.005% sulfur, and thebalance essentially nickel.

5. The process according to claim 1 wherein the alloy contains about 14%to about 16% chromium, about 2.75% to 3.25% columbium, about 2.75% to3.25% tungsten, about 2.75% to 3.25 molybdenum, about 5% to 9% iron, upto about 0.08% carbon, about 0.3% to 0.6% aluminum, about 0.4% to 0.7%titanium, about 0.003% to about 0.008% boron, about 0.01% to about 0.05%zirconium, up to about 0.75% manganese, up to about 0.4% silicon, notmore than 0.005% sulfur, and the balance essentially nickel.

6. The process according to claim 1 wherein the alloy contains about 20%to 22% chromium, about 1.5% to about 2.5% columbium, about 3.5% to 4.5%molybdenum, up to about 2% iron, up to about 0.07% carbon, about 0.2% to03% aluminum, about 2.3% to 3% titanium, about 0.002% to 0.008% boron,about 0.02% to about 0.08% zirconium, up to about 0.25% manganese, up toabout 0.15% silicon, not more than 0.005% sulfur, and the balanceessentially nickel.

References Cited UNITED STATES PATENTS 2,570,193 10/1951 Bieber et al.75171 6 3,160,500 12/1964 Eiselstein et al; 75-171 3,304,176 2/1967Wlodek 75-171 3,383,204 5/1968 Hagel 75-170 US. Cl. X.R.

Patent No. 315121953 Dated i 1970 Inventor (a) a It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 2, line 46, in the 6th Column of Table I, last number,

change "2.08" to -2.8-. In the footnote of Table I, for

"boro-" read "boron,".

Col. 3, after Table III, insert the footnote 'Held in vacuum afteraddition of 0.06% magnesium-. Same column, line 65, for

"treated" read --treatment-. Also, line 68, for "on" read Signed andsealed this 29th day of June 1971.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

